1. Field of the Invention
The present invention is related to a communication system and, more particularly, to a pulse code modulation communication system utilized for the remote control of flying model aircraft.
2. Description of the Prior Art
In remote control radio systems utilized for flying a model aircraft it is necessary to position the flight control surfaces of the aircraft in response to commands from the ground. Further, it is necessary to position such control surfaces in a manner directly related to the movement of a control stick, or a plurality of such control sticks, operated by the ground-based pilot. The mechanism utilized to position the flight control surfaces in the aircraft is commonly referred to as a servo which generally comprises an electromechanical unit including a motor whose output is controlled by command voltage inputs.
Prior art remote control systems utilized for controlling the flight of model aircraft have generally been of an analog nature wherein a series of pulses, each corresponding to a different control command, are utilized to modulate an RF carrier wave operating at a particular frequency. In such a system, while the frame rate is fixed, the time interval between any two successive pulses within a frame may be varied. It is this variable width between individual pulses that is utilized to position the servo motors, one of each of the variable width segments being utilized to control each servo. A decoder in the receiver passes the first pulse to the first servo unit, the second pulse to the next servo unit, and so forth, such that each servo unit receives one pulse from each frame, the length of the pulse being determinative of the position of the corresponding servo.
While such analog systems have been generally acceptable, they suffer from certain inherent deficiencies such as a lack of linearity in the servo output and susceptibility to random noise. Further, each radio control system of this type must be constructed to operate at a different RF carrier frequency to avoid adjacent system interference.